//翻转链表
//给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。
//循环双指针
struct ListNode* reverseList(struct ListNode* head) {
	if (!head)
		return NULL;
	struct ListNode* cur = head;
	struct ListNode* nnext = NULL;
	while (head->next) {
		nnext = head->next->next;
		head->next->next = cur;
		cur = head->next;
		head->next = nnext;
	}
	return cur;

}